Solution and method for producing heat resistant electrical insulation coatings on ferrous surfaces



SOLUTION AND FOR PRODUCING HEAT RESISTANT ELECTRICAL INSULATION COATINGS N FERRGUS SURFACES Heinz Keller, Karl Lampatzer, and Rudolf Brodt, Frankfurt am Main, Germany, assignors to Parker Rust Proof Company, Detroit, Mich, a corporation of Michigan No Drawing. Application March 9, 1955 Serial No. 493,308

Claims priority, application Germany March 9, 1954 6 Claims. (Cl. 148-616) The present invention relates to a solution and method for producing heat-resistant electrical insulation coatings on ferrous surfaces. More particularly this invention is concerned with the formation of electrical insulation coatings on roll or strip steel which are capable of withstanding the temperatures which are necessary for imparting magnetic properties to the strip steel after it has been coated.

In the production treatment of wires, bands, sheet steel and the like, conventional processing includes the sequential steps of degreasing, derusting or descaling, phosphate coating, rinsing and drying. Such processing steps requiring the storing, regenerating and pumping of large quantities of treating and rinsing solutions. The disposal of wastesolutions of this type represents a substantial commercial problem to those handling such solutions. It is one of the objects of the present invention to substantially reduce and at least partially eliminate the necessity for disposing of large quantities of such Waste materials and this is accomplished by providing coating methods which utilize dry processing steps with the exception of the phosphate coating step.

In the processing of strip steel for electrical and magnetic uses it is desirable to preliminarily coat the strip or Wire material with a protective phosphate coating for the purpose of protecting the metal surface during forming operations of the strip'orwireinto the final desired shape, and to thereafter heat treat the formed object to provide the necessary electrical or magnetic properties. The use of heretofore known conventional phosphating solutions has been unsuccessful in providing phosphate coatings which would satisfactorily withstand the necessary temperatures for obtaining the desired electrical or magnetic propertiesin the coated metal due to the breakdown of the coating as the temperature was raised to temperatures above about 1000" C.-1100 C. It is therefore the primary object of the present invention to provide a solution and method for forming protective phosphate coatings on ferrous surfaces which are capable of simultaner ously resisting the high temperatures necessary to impart to ferrous surfaces the desired electrical or magnetic prop erties after the protective coating has been applied to the surface of the metal part.

U. S. Patent No. 1,428,087 proposes the use of thermally decomposable phosphates such as ammonium or aniline phosphates for the production of phosphate coatings on metallic surfaces as the result of the release of phosphoric acid upon the application of heat to the metal surface coated with the phosphate. A somewhat similar disclosure is found in U. S. Patent No. 1,805,982 in which thermally decomposable phosphates are proposed in combination with neutralizing agents such as iron oxide. In accordance with the present invention it has now been found that the above and -related-objects may be completely accomplished by employing as the coating producing material an aqueous or alcoholic or mixed States Patent aqueous-alcoholic solution of a thermally decomposable phosphate in combination with a heat-resistant filler material which has the capacity to resist melting at temperatures of approximately 1000 C. The thermally decomposable phosphate may vary rather Widely in its actual make-up and the only requirements which must be satisficd are that the phosphate material is capable of being thermally decomposed to release phosphoric acid upon heating and that the disassociated cation which is thereby released is either volatile or is not detrimental to the coating which is formed. Numerous specific phosphate materials falling into the above generally defined category are set forth in greater detail below. The heatresistant filler materials which contribute to the resultant coating its markedly increased heat resistance without concurrently destroying or detrimentally affecting its electrical insulating properties are silicic acid, mica, magnesium oxide, aluminum oxide and hydrated aluminum silicate clays, such for example as bentonite or kaolin or china clay. These filler materials are preferably in the form of finely ground powdered materials and have been observed to serve as the separating layer or insulating layer between adjacent sheets of coated strip steel at temperatures as high as 1000" C. in a hydrogen atmosphere or approximately 850 C. in air even when the sheets are subjected to relatively high load.

Particularly suitable for the purposes of this invention are the ammonium phosphates and substituted ammonium phosphates. In this specification and in the appended claims the term substituted ammonium phosphates is intended to mean aliphatic substituted ammonium phosphates in which the length of the carbon chain in any substituent group contains less than 7 carbon atoms, the substituerit groups being alkyl or aryl. Typical examples of substituted ammonium phosphates are 'isopro'pylamine phosphate, mohoethanolamine phosphate, 'cyclohe'xylami'ne phosphate, 'di-n-emylamine phosphate, etc. Aniline phosphate, dianiline phosphate and materials such as hrea phosphate and morpholine phosphate may also be satisfactorily employed. Pyridine phosphates are similarly useful. The phosphoric acid esters whosecation components pass off as alcohol upon heating or which areconsumed and do not'disturb the formation or nature of the resultant coating are particularly suitable 'for this invention and this specifically includes the methyl, ethyl, propyl' and butyl, phosphate esters. Similarly di esters and mixtures of monoand di-esters may be employed so long as the carbon atoms in the esters do not exceed '8; in using the monoesters, typical examples sr satisfactory esters are the methyl, ethyl, propyl, butyl, isopropyl an'dptertiary butyl phosphate esters and the corresponding di-esters. Additionally, any of the mixed esters, such for example as methylethyl, methyl-pr'opyl, ethyl-propyl, ethyl-butyl phosp'hates', etc, may be employed. Ammonium substituted esters are also intended to be included, for example, the ammoniummethyl, ammonium-ethyl, ammonium-propyl phosphate esters, etc.

The operating solutions of this invention may satisfactorily contain awidevariation in the concentrations of the coating-producing ingredients and satisfactory limits are broadly defined as follows: 200400 grams/liter of the phosphate ion introduced as ammonium dihydrogen phosphate, or its equivalent as above set forth, l070 grams/liter of a filler material selected from the group consisting of silicic acid, mica, magnesium oxide, aluminum oxide, and hydrated aluminum silicate clays and a solvent selected from water, alcoholandmixtu'res ofwater and alcohol. If desired, the solution as above defined may also include 5-50grams/literofa-chrornic acid derivative such as the ammonium-or substituted ammonium derivatives containing less than 7 carbon atoms in any one substituent group. For those applications requiring the highest heat resistance it has been found to be desirable to include in the above compositions a proportion of the calcium ion or the barium ion equivalent to l2-26 grams/liter of calcium. The calcium may be introduced as the mono-calcium phosphate, di-calcium phosphate, calcium monohydrogen phosphate, etc. and the barium as barium dihydrogen phosphate or barium monohydrogen phosphate. When barium is employed it is advisable to incease the acidity of the solution somewhat and this can be done by adding urea phosphate, for example. Coatings which result from the application of solutions containing the calcium or barium ion have somewhat superior heat resistance and chemical resistance to those obtained from solutions in the absence of the calcium or barium ion. The calcium or barium apparently becomes a part of the coating and during the burning-in or heating after the application of the coating solution a reaction apparently occurs between the ferrous surface and the calcium phosphate portion of the solution which enhances the adherence and heat resistance of the resultant coating. The 200 grams/liter minimum of P may be reduced proportionally when calcium or barium are used.

The method of this invention comprises the steps of contacting the preliminarily cleaned ferrous surface with the solutions of this invention either by immersion or by spraying. Immediately after the coated surface has been withdrawn from contact with the coating solution the surface is heated to a temperature in the range of 250 C. to 800 C. and preferably 400 C. to 500 C. The thermally decomposable phosphates, upon decomposition release a gaseous product which during the short heating cycle is suflicient to protect the surface from oxygen and the resultant coating is substantially free of oxide. One of the important commercial features of the present invention is that the heating step may be accomplished in air, thereby avoiding the necessity for providing expensive controlled atmosphere heating devices. In the above defined coating solutions it is satisfactory to contact the ferrous surface for a very short period of time with the coating solution, for example, from approximately 2 to 30 seconds, and to thereafter heat the coated surface for 10 to 120 seconds, depending upon the burning-in temperature. Upon cooling, the resultant coating is completely formed and it is unnecessary to rinse or otherwise treat the coated surface. In some applications it has been found to be desirable to slightly shape the coating after cooling, as for example, by means of cold rolling in which a slight reduction is effected, and by this procedure the coating is leveled and the surfaces become extremely smooth and compact. Such surfaces have been found to be especially chemically resistant and to satisfactorily resist the attack of both acid and alkalies.

The solutions of this invention and the method of this invention are particularly well suited for use in connection with the continuous fabrication of electrical insulation steel parts such as motor laminations, transformer cores and the like. In such operations it is feasible to employ a preliminary oxidation or scale forming step followed by a reducing atmosphere cleaning and cooling prior to the introduction of the strip material into the coating solutions of this invention. It will be apparent that the present process makes it possible to eliminate conventional rinsing steps and the regeneration or disposal treatments applied to waste materials which have heretofore been employed in continuous processes. Additionally the present process makes it possible to omit the preliminary wet cleaning operations including degreasing, pickling, rinsing, neutralizingetc. prior to the actual phosphate coating step. Where the present process is employed as a part of the continuous production of electrical insulation strip steel or strip steel having peculiarly desired magnetic properties, it is desirable to bring the strip material back to room temperature prior to its introduction into the coating solutions of this invention although it is to be understood that the strip material may be introduced into the coating solution at a somewhat elevated temperature if desired.

The examples given below will serve to illustrate the compositions and the method of this invention in greater detail. It is to be understood, however, that the examples set forth below are merely typical examples of satisfactcry operating solutions within the broad range of proportions of ingredients set forth above. The specific concentrations and the specific materials illustrated in the examples are not to be considered the exclusively operative materials for the purposes of this invention but rather merely illustrative of the materials and concentrations broadly set forth above.

Example 1 An aqueous solutions was prepared containing grams/liter of ammonium dihydrogen phosphate, 140 grams/ liter of calcium orthophosphate (technically pure), 30 grams per liter of primary urea phosphate and 30 grams/liter of powdered mica (200 screen mesh), water to make one liter. Preliminarily cleaned cold-rolled silicon steel, containing approximately 2% silicon, for use in German transformer cores, was contacted by the above solution both by spraying and dipping with a contact period of approximately 2 to 5 seconds. The strip, upon emerging from the treating solution was immediately raised to a temperature of 500 C. and maintained at that temperature for 60 to 100 seconds and allowed to cool. The resultant coating was found to be adherent, uniform, and to have a weight varying from about 100 to 180 mg./sq. dm. The electrical insulation properties of the coating were measured in accordance with the following test. A measuring electrode having a measuring surface of 1 square centimeter is positioned on the coated surface to be tested. An adjustable measuring voltage is attached to the coated side with an appropriate electrode contacting the opposite side of the coated sheet. The voltage is applied and increased until a breakdown and passage of a current through the coating and metal is obtained. In accordance with this testing procedure, the coating resulting from the above treatment was found to break down at 60 volts applied when the pressure or weight on the coating was 15 kg./ sq. cm. The coated surface was then annealed at a temperature of 850 C. for 30 minutes under a protective nitrogen atmosphere and the voltage break down test applied to the annealed material. It was found that the annealed material did not break down until a voltage of 40 volts was applied under a similar load of 15 kg./sq. cm.

Example 2 An aqueous solution was prepared, and upon analysis found to contain 300 grams/liter of ammonium dihydrogen phosphate, 10 grams/liter ammonium chromate and 20 grams/liter of finely ground silicic acid. Cold rolled steel strip in a relatively cleaned condition was preliminarily heated at about 750 C. under a protective gas atmosphere and then cooled down to about 300 C. under the same protective gas atmosphere. The strip was then permitted to cool to room temperature in air and a slight scale was formed. The steel strip was then contacted with the above solution for a period of approximately 2 seconds by passing the strip through a tank containing the above solution at a speed of about one meter per second. Immediately upon emerging from the solution the temperature of the strip was raised to between 450 C. and 500 C. and maintained at that temperature for about twenty seconds. The strip emerging from the coating solution passed between a pair of smoothing rollers which were adjusted to leave a film thickness of approximately 10 to 50 grams/square meter on the surface which, after heating, produced a coating having a thickness of about 3 /3 to 16 /3 grams per square meter. The coating was found to be uniform and of unusually high adherence to the surface, the weight of the coating being approximately twice that which is obtained from conventional zinc phosphate-zinc nitrate solutions. The coating was found to have unusually good corrosion resistance and to be a suitable base for the application oflacquers or the like, and to have especially good electrical insulation values even after annealing of the steel strip at temperatures within the range of 800 C. to 850 C.

Example 3 An aqueous solution was prepared and analyzed to have the following constituents: 300 grams/liter ammonium dihydrogen phosphate, 200 grams/liter ammonium hydrogen phosphate, 40 grams/liter silicic acid in powdered form and 0.5 gram/liter of dodecylbenzolsulphonate. Cold rolled steel strip after preliminary cleaning operations was passed through the above solution at a speed of 2 meters per second and upon Withdrawal from the solution was passed through an oven having a temperature of 500 C. and maintained in that oven for 30 seconds. After the coating was allowed to cool, the coated metal was drawn between rollers to produce a 3% thickness reduction. The resulting strip was inspected and found to have on its surface an even, firm, uniform surface having good electrical resistance properties as well as good rust resisting properties and to be capable of being painted without further alteration.

The following bath compositions are given to illustrate additional compositions which, when employedin accordance with the procedural steps set forth in the above examples, have been found to be satisfactory for the purposes of this invention.

Example 4 400 grams/liter of urea phosphate grams/liter powdered mica 0.5 gram/liter triethanol ammonium dodecylbenzolsul phonate water to make a liter Example 5 300 grams/liter monomethyl ester of phosphoric acid 40 grams/liter bentonite Sufiicient water-alcohol mixture (50-50) to make a liter What is claimed is:

1. A composition of matter comprising a dispersion consisting essentially of 200400 grams/liter of at least one thermally decomposable phosphate selected from the group consisting of ammonium phosphates, aliphatic substituted ammonium phosphates having less than 7 carbon atoms in any substituent group, monoesters, diesters and mixed mono and di-esters of phosphoric acid containing not more than 8 carbon atoms, 10-70 grams/liter of a heat resistant material selected from the group consisting of silicic acid, mica, hydrated aluminum silicate clays, magnesium oxide and aluminum oxide, 550 grams/ liter of at least one thermally decomposable chromate selected from the group consisting of ammonium chromates and aliphatic substituted ammonium chromates having less than 7 carbon atoms in any substituent group, 12-26 grams/liter of an ion selected from the group consisting of the calcium ion and the barium ion, and a solvent selected from the group consisting of water, alcohol and mixtures thereof.

2. A composition of matter comprising a disperson consisting essentially of 200-400 grams/liter of at least one thermally decomposable phosphate selected from the group consisting of ammonium phosphates, aliphatic substituted ammonium phosphates having less than 7 carbon atoms in any substituent group, monoesters, diesters and mixed monoand di-esters of phosphoric acid containing not more than 8 carbon atoms, 10-70 grams/ liter of a heat resistant material selected from the group consisting of silicic acid, mica, hydrated aluminum 6 silicate clays, magnesium oxide and aluminum oxide, and 12-26 grams/literof the calcium ion, and a solvent selected from the group consisting of water, alcohol and mixtures thereof.

3. A composition of matter comprising a dispersion consisting essentially of 200-400 grams/liter of at least one thermally decomposable phosphate selected from the group consisting of ammonium phosphates, aliphatic substituted ammonium phosphates having less than 7 carbon atoms in any substituent group, monoesters, diesters and mixed monoand di-esters of phosphoric acid containing not more than 8 carbon atoms, 10-70 grams/ liter of a heat resistant material selected from the group consisting of silicic acid, mica, hydrated aluminum silicate clays, magnesium oxide and aluminum oxide, and 12-26 grams/liter of an ion selected from the group consisting of the calcium ion and the barium ion, and a solvent selected from the group consisting of water, alcohol and mixtures thereof.

4. A method for forming an electrical insulation coating .on ferrous surfaces which comprises the steps of contacting a ferrous surface with a dispersion consisting essentially of 200-400 grams/liter of at least one thermally decomposable phosphate selected from the group consisting of ammonium phosphates, aliphatic substituted ammonium phosphates having less than 7 carbon atoms in any substitutent group, mono-esters, di-esters and mixed monoand di-esters of phosphoric acid containing not more than 8 carbon atoms, 10-70 grams/liter of a heat-resistant material selected from the group consisting of silicic acid, mica, hydrated aluminum silicate clays, magnesium oxide and aluminum oxide, 5-50 grams/liter of at least one thermally decomposable chromate selected from the group consisting of ammonium chromates and aliphatic substituted ammonium chromates having less than 7 carbon atoms any substituent group, 12-26 grams/liter of an ion selected from the group consisting of the calcium ion and the barium ion, and a solvent selected from the group consisting of water, alcohol and mixtures thereof, removing said ferrous surface from said solution and thereafter heating said surface to a temperature in the range of 250 C. to 800 C. and maintaining said temperature for a time suflicient to thermally decompose the said phosphate on said surface and to form a protective phosphate coating thereon.

5. A method for forming an electrical insulation coating on ferrous surfaces which comprises the steps of contacting a ferrous surface with a dispersion consisting essentially of 200-400 grams/liter of at least one thermally decomposable phosphate selected from the group consisting of ammonium phosphates, aliphatic substituted ammonium phosphates having less than 7 carbon atoms in any substituent group, mono-esters, di-esters and mixed monoand di-esters of phosphoric acid containing not more than 8 carbon atoms, l0-70 grams/liter of a heat-resistant material selected from the group consisting of silicic acid, mica, hydrated aluminum silicate clays, magnesium oxide and aluminum oxide, and 1226 grams/liter of an ion selected from the group consisting of the calcium ion and the barium ion, and a solvent selected from the group consisting of water, alcohol and mixtures thereof, removing said ferrous surface from said solution and thereafter heating said surface to a temperature in the range of 250 C. to

' 800 C. and maintaining said temperature for a time sufiicient to thermally decompose the said phosphate on said surface and to form a protective phosphate coating thereon.

6. A method for forming an electrical insulation coating on ferrous surfaces which comprises the steps of contacting a ferrous surface with a dispersion consisting essentially of 200-400 grams/liter of at least one thermally decomposable phosphate selected from the group consisting of ammonium phosphates, aliphatic sub- 7 V stitutecl ammonium phosphates having less than 7 carbon atoms in any substituent group, mono-esters, di-esters and mixed monoand di-esters of phosphoric acid containing not more than 8 carbon atoms, 10-70 grams/ liter of a heat-resistant material selected from the group consisting of silicic acid, mica, hydrated aluminum silicate clays, magnesium oxide and aluminum oxide, and 12-26 grams/liter of the calcium ion, and a solvent selected from the group consisting of water, alcohol and mixtures thereof, removing said ferrous surface from said solution and thereafter heating said surface to a temperature in the range of 250 C. to 800 C. and maintaining said temperature for a time suflicient to thermally decompose the said phosphate on said surface and to form a protective phosphate coating thereon.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A COMPOSITION OF MATTER COMPRISING A DISPERSION CONSISTING ESSENTIALLY OF 200-400 GRAMS/LITER OF AT LEAST ONE THERMALLY DECOMPOSABLE PHOSPHATE SELECTED FROM THE GROUP CONSISTING OF AMMONIUM PHOSPHATES, ALIPHATIC SUBSTITUTED AMMONIUM PHOSPHATES HAVING LESS THAN 7 CARBON ATOMS IN ANY SUBSTITUENT GROUP, MONOESTERS, DIESTERS AND MIXED MONO AND DI-ESTERS OF PHOSPHORIC ACID CONTAINING NOT MORE THAN 8 CARBON ATOMS, 10-70 GRAMS/LITER OF A HEAT RESISTANT MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICIC ACID, MICA, HYDRATED ALUMINUM SILICATE CLAYS, MAGNESIUM OXIDE AND ALUMINUM OXIDE, 5-50 GRAMS/ LITER OF AT LEAST ONE THERMALLY DECOMPOSABLE CHROMATE SELECTED FROM THE GROUP CONSISTING OF AMMONIUM CHROMATES AND ALIPHATIC SUBSTITUTED AMMONIUM CHROMATES HAVING LESS THAN 7 CARBON ATOMS IN ANY SUBSTITUENT GROUP, 12-26 GRAMS/LITER OF AN ION SELECTED FROM THE GROUP CONSISTING OF THE CALCIUM ION AND THE BARIUM ION, AND A SOLVENT SELECTED FROM THE GROUP CONSISTING OF WATER, ALCOHOL AND MIXTURES THEREOF. 